氮气开关流柱形成过程的理论研究

Theoretical study on the stream formation in the nitrogen switch

使用蒙特卡罗-粒子模拟方法对氮气开关中的流柱形成过程进行模拟,并结合计算结果对其进行理论分析.发现在流柱击穿发生前(即空间电荷场远小于本底电场),等离子体的电离频率、电子平均能量及其迁移速度等都近似为常数,因此可以解析求解电子数密度方程对等离子体的演化行为进行分析.在击穿发生后,随机碰撞过程会破坏初始等离子体区域分布的对称性,并出现分叉的等离子体区域结构.在放电过程中,随着等离子体密度增加,其内部基本保持电中性且电场不断减小,靠近阴阳极两端电荷分离产生的净电荷密度不断增加,场强也不断增加,且靠近阳极端的电荷密度(绝对值)和场强都大于阴极端.通过改变极板间电压发现,平均电子能量随极板间场强增加而增加,电子迁移速度随着场强近似线性增加,电离频率随场强的变化快慢介于E^4与E^5之间.

The stream formation in a 1-atm nitrogen gas switch is investigated by the two-dimensional and three-velocity（2D3V） particles through the cell-Monte Carlo collision（PIC-MCC） simulation and theoretical analysis.For simplicity,two parallel plane electrodes of 0.6 mm width are separated by a distance of 1.6 mm.It is found that the analytical solution of the electron density equation can be used to study the evolution of the plasma before the stream breaks down,for the ionization frequency,mean electron energy and electron drift velocity are all constant.After the breakdown of the stream,random collisions destroy the symmetry of the plasma region and cause plasma to branch.As plasma density increases,the electric field inside the plasma region decreases due to the shielding effect.However,charge densities at both ends of the plasma region increase and the density at the anode end is larger than that at the cathode end,for the plasma exponentially grows as electrons move from the cathode toward the anode.This causes the electric field at the end of plasma near the anode to be larger than that near the cathode.It is found that the electrons can achieve their stable mean energy in several picoseconds due to the high transfer frequency（10^（11）-10^（12） Hz） of the electron energy in the nitrogen plasma.After the breakdown of the stream,the mean electron energy decreases due to the decrease of the electron energies inside the plasma.By increasing the electrode voltage,it is found that the mean electron energy increases,the electron drift velocity increases linearly,and the variation rate of ionization frequency with electric field is in a range between E^4 and E^5.Therefore,the time taking for breaking down the stream decreases with the increase of the electrode voltage.